The nature and chemistry of synthetic polymers used in biomedical applications varies depending on the application. In general they need to have the advantage over other materials of being able to be fabricated into any shape, and the ability to be tailored as far as their mechanical properties are concerned to the application at hand. Such polymers can also be tailored to contain various functionalities allowing them to integrate into the environment in which they are to be placed. Some implantable polymers for example are biodegradable and degrade to fragments that can be easily resorbed through metabolic pathways that the body uses to cleanse itself of undesired by products.
Synthetic biomedical polymers can be broadly classified in to non-biodegradable and biodegradable polymers. Those that are non-biodegradable are widely used when a medical device needs to be in place indefinitely or until such time as it is decided that the device is no longer required and can be safely removed, i.e. in permanent fixation devices. These polymers need to be completely non-biodegradable or have minimal degradation properties in the environment in which they are placed, and are, for example, widely used in areas such as breast implants, in orthopaedic applications such as bone fixation devices and more recently, to replace important tissues such as heart valves.
Polysiloxanes, polyurethanes and or their copolymers are widely used in such applications.
A vast majority of biodegradable polymers studied belong to the polyester family. Among these poly(α-hydroxy acids) such as poly(glycolic acid), poly(lactic acid) and a range of their copolymers have historically comprised the bulk of published material on biodegradable polyesters and have a long history of use as synthetic biodegradable materials in a number of clinical applications. Among these applications, poly(glycolic acid), poly(lactic acid) and their copolymers, poly(p-dioxanone), and copolymers of trimethylene carbonate and glycolide have been the most widely used. Their major applications include resorbable sutures, drug delivery systems and orthopaedic fixation devices such as pins, rods and screws. Among the families of synthetic polymers, the polyesters have been attractive for use in these applications because of their ease of degradation by hydrolysis of their ester linkage, the fact that their degradation products are resorbed through metabolic pathways in some cases and their potential to be tailored in terms of their structure to alter degradation rates.
Almost all of these polymers, both biodegradable and biostable, are pre manufactured or pre-cured and moulded, prior to application. In a minority of cases two or many different reactive species are mixed together immediately prior to application, so that the user has a specified window of time before the polymer begins to set or cure and becomes unworkable. In bone fixation devices a two part mixture can be applied or injected to the specific site following mixing but before it cures to a hard substance that supports the defect. Other hybrid materials which are mixtures of inorganic substances such as ceramics and polymers have been extensively investigated for orthopaedic repair. These polymer systems again rely on the use of uncontrolled curing by mixing a two part system to obtain the final cured solid material. Although these have the advantage of prefabricated pore sizes and superior mechanical strength, they suffer from an inability to be shaped to the required geometry.
Polymer compositions that can cure by multiple curing methods and steps which are designed for applications such as surface coatings and adhesives are reported in the literature. However, such compositions are not suited for biomedical applications because of their highly complex nature arising from the presence of numerous compounds incorporated to meet various product requirements.
It is one object of this invention to provide multifunctional polymer compositions for use in biomedical applications, the curing process of which may be controlled to allow ease of use, and the design of which may be adjusted to suit the specific application. The polymers are desirably capable of incorporating biological components such as cells, growth factors and other components such as nano-particulate hydroxyapatite, calcium phosphate or other particles which maybe an adjunct in the application selected.